This invention relates to methods for detecting the failure of nuclear fuel elements, and more particularly it deals with a method for detecting the failure of nuclear fuel elements wherein the degree of precision with which the failure of any of the nuclear fuel elements is detected can be increased by supplying warm water to the interior of a nuclear fuel assembly.
Generally, the nuclear fuel elements constituting each of the fuel assemblies arranged in a pressure vessel of a nuclear reactor have the hazard of failing when used for a prolonged period of time. From the standpoint of safety of operation of the nuclear reactor, it is not desirable to use fuel assemblies containing faulty fuel elements, because radioactive materials are released from the faulty fuel elements into cooling water which functions as coolant, with the result that nuclear fission fragments in the cooling water increase in amount. Thus, the present practice requires inspection of the fuel assemblies at regular intervals, and fuel assemblies containing faulty fuel elements are replaced by new ones.
In a boiling-water reactor, detection of faulty nuclear fuel elements is started in about 5 to 7 days after the nuclear reactor is shut down. This is because the reactor pressure vessel has a high concentration of radioactivity and it takes a considerable number of days for the concentration to be lowered. Also, it is a time-consuming operation to remove the top cover of the reactor pressure vessel, the drier in the reactor pressure vessel and the steam separator after the concentration of radioactivity in the pressure vessel is lowered. Moreover, the need to provide a pool for water over the reactor pressure vessel is one of the factors concerned in a delay in initiation of an operation for detecting faulty fuel elements.
A plurality of nuclear fuel assemblies are arranged in the core of a reactor pressure vessel. Even after shutdown of the nuclear reactor, a current of cooling water serving as coolant is constantly supplied to the reactor pressure vessel and flows through the fuel assemblies to remove decay heat generated by the nuclear fuel elements constituting each fuel assembly. The cooling water heated by the decay heat is led out of the reactor pressure vessel, cooled and returned to the reactor pressure vessel.
A method of the prior art for detecting faulty nuclear fuel elements in a boiling-water reactor will first be described. The fuel assemblies arranged in a reactor pressure vessel are located such that their upper ends are disposed about 10 meters below the liquid surface of a pool of cooling water over the reactor pressure vessel. The cooling water in the aforesaid pool and the reactor pressure vessel is hereinafter referred to as reactor cooling-water. Each nuclear fuel assembly comprises a channel box and a plurality of nuclear fuel elements arranged therein. The fuel assemblies are mounted in a reactor core in the pressure vessel. The top of each fuel assembly is covered with a cap having an air supply line and a sampling pipe extending therethrough. Air is supplied to the interior of the cap through the air supply line to form an air layer in the cap. The provision of this air layer results in the formation of a liquid level in the upper portion of the fuel assembly.
The formation of the liquid level in the fuel assembly results in the blocking of the passage of cooling water through the fuel assembly, so that decay heat generated by the fuel elements is not removed completely. Thus, the temperature in the fuel assembly rises and, if there are faulty fuel elements, fission products will be released into the cooling water through the damaged portions. The cooling water is also heated, and the fission products released into the heated cooling water are dissolved therein and dispersed by natural convection within the fuel assembly. After the lapse of scores of minutes following the mounting of the cap on the top of the fuel assembly, a sample of the cooling water in the upper portion of the fuel assembly is taken through the sampling pipe and radioactivity thereof is measured. Judgement of whether there are faulty fuel elements in the fuel assembly is passed on the basis of the radioactivity determined as the result of the test.
The aforementioned method of the prior art for detecting faulty nuclear fuel elements has some disadvantages. First, since it is after 5 or 7 days have elapsed following shutdown of the nuclear reactor that an operation for detecting faulty nuclear elements is performed, the amount of heat in the cooling water generated by decay heat of the fuel elements is small, and the fission products released from the faulty portions of the fuel elements are very small in amount, when tests are performed. More specifically, if the internal pressure of a nuclear fuel element became higher than its external pressure due to a rise in the temperature of the fuel element, then fission products would be released from the fuel element to outside. However, since 5 to 7 days have elapsed after the nuclear reactor is shut down when tests are performed, the internal pressure of the fuel element has been reduced to a level at which it is substantially the same as its external pressure. Accordingly, the fission products released from the fuel elements are very small in amount and consequently the degree of precision with which the existence of faulty fuel elements is detected is very low. The released fission products are dispersed in the cooling water in the fuel assembly due to natural convection of the cooling water in the fuel assembly. Owing to the fact that a sample of the cooling water is taken from the water in the upper portion of the fuel assembly, there is no alternative but to wait for the released and dissolved fission products to reach the upper portion of the fuel assembly by natural convection, which is time-consuming. Particularly, when the failure of a fuel element or elements occurs in the lower portion of the fuel assembly, difficulty is encountered in detecting the existence of a faulty fuel element or elements because of the facts that the fission products released into the cooling water are very small in amount and that it takes a long time for the fission products to reach the upper portion of the fuel assembly.